1. Field of the Invention
The present invention relates to a method to coarse-tune the center frequency of integrated VCO(Voltage Control Oscillator), and more particularly, to a two-step VCO calibration method.
2. Description of the Related Art
In SOC (system on chip) field, all the functional blocks should be integrated to a single chip. Therefore the layout size becomes a critical issue in circuit design. Generally phase lock loop (PLL) block design contains VCO, divider, phase/frequency detector, charge-pump and passive loop filter, and among this block, passive loop filter will occupy the largest area if a medium loop band-width is selected. In order to reduce the die area due to this large capacitor for providing lower frequency pole, the gain of VCO must be lower to afford less use of passive capacitor. On the other hand once the gain of VCO lower, there is no margin for temperature variation of integrated VCO. So, there is the technology of coarse-tuning the center frequency of integrated VCO, which is used to reduce the size of passive loop filter. The smaller the size of passive loop filter is, the easier integrating the loop filter becomes.
In the prior arts, there are two ways to coarse-tune the center frequency of integrated VCO, one is to calibrate the frequency at power-on, the other is to calibrate the frequency at transmit/receive data packets.
However, the first way, calibration at power-on, can cover process variation only. Once the operation temperature varied to high or low, this variation should be recovered by analog PLL. This will lead a large requirement of charge-pump dynamic range. In addition, if the VCO gain isn't flat enough the loop band-width of PLL will be varied by temperature.
The other way, calibration at transmit/receive data packet, is a better method to cover all the variation of process and temperature. Nevertheless, due to the calibration must be done before packet start, an extra and long calibration time should be needed at each packet resulting in the increase of power consumption.
According to the above problems, the related field needs a new method to calibrate the center frequency of integrated VCO to overcome the disadvantages of the prior arts.